This invention relates to a digital system for detecting mechanical movement of a body under detection to produce a digital signal indicative of the position or the speed of the body or to produce pulses in synchronism with the movement.
In the past, a tachometer generator having the same construction as a DC generator was used for detecting the rotational speed of a rotating body. Position of a rotating body was detected by means of a potentiometer coupled to the shaft of the body. Detection of the speed or the position of linear movement of a body was achieved by converting the linear movement into rotation by means of gears or the like, and by detecting the rotation in a similar manner.
The signals indicative of the speed and the position thus detected are often used as feedback signals. When the feedback control system is composed of analog circuits, the analog signals from the tachometer generator are convenient.
However, in recent years, control systems often comprise digital circuits including microcomputers, use of which makes it possible to improve accuracy and stability of control. In addition, a digital system has a smaller number of devices which require adjustment and use of a microcomputer reduces the cost of the system.
When a digital control system is used, a feedback signal must be in the form of a digital signal. One way of providing a digital signal is to convert an analog feedback signal into a digital signal by means of an analog-to-digital converter. But, the original analog signal is less accurate and less stable, so that advantages of using a digital system are reduced. Moreover, tachometer generators and potentiometers are associated with mechanically sliding parts, which require frequent inspection and maintenance, which are troublesome and costly.
In another conventional speed detection system, a rotary pulse generator or a frequency generator generating an output of a frequency proportional to the rotational speed is used. The number of pulses generated is counted and a digital signal indicative of the detected speed is produced. This system has a disadvantage in that detection with high accuracy cannot be achieved unless the number of pulses generated during one revolution is very large. For instance, when the rotational speed of 960 rpm (=16 rps) is to be detected at a sampling period of 10 m sec. and with a resolution of 12 bits, i.e., about 0.025% (=2.sup.-12), a pulse generator producing 25,600 pulses per revolution is required. This means the output of the pulse generator is more than 400 KHz when the detected speed is 960 rpm, and transmitting a pulse signal of such a high frequency from the pulse generator to a control device is not easy. Moreover, such a pulse generator will have very delicate construction, and is therefore difficult to handle, and is costly.